Charge coupled device imagers formed on a semiconductor substrate are described in Wolfe et al, The Infrared Handbook, Office of Naval Research, Department of the Navy, 1978, pp. 12-27 to 12-54. They typically comprise an array of parallel charge coupled device (CCD) serial registers receiving charge from adjacent photodetectors on the substrate. Typically, the parallel CCD registers are arranged in vertical columns, all of their outputs being connected to a single horizontal CCD register which serves as the multiplexer. All of the column CCD registers are connected to a common input diffusion and a common input gate which separately introduces a FAT zero charge packet into each of the column CCD registers.
This type of imager suffers from several inherent limitations. Specifically, electronic noise or jitter on the signals controlling the injection of the FAT zero charge packet from the input diffusion into each of the column registers causes the amount of FAT zero charge in each of the packets to vary, which in turn generates row correlated noise. Specifically, electronic jitter in the clock signals controlling the input diffusion and the adjacent overlying input gate electrodes causes the capacity of each CCD bucket defining the amount of FAT zero charge to vary in synchronism with the jitter.
Another limitation is the presence of thermal noise generated by the behavior of the semiconductor material. For example, semiconductor surface states at the interface between the semiconductor substrate and the overlying dielectric layer causes the amount of charge carriers contained in each CCD bucket to vary, which in turn generates uncorrelated thermal noise in the video signal.
The total amount of CCD noise from these two sources, N.sub.CCD, is equal to the square root of the sum of the squares of the row correlated noise, N.sub.RC, and the thermal noise, N.sub.TH, in accordance with a well-known principle, which may be expressed as: EQU N.sub.CCD =(N.sub.RC.sup.2 +N.sub.TH.sup.2).sup.1/2.
Another limitation is that the limited charge capacity of each CCD bucket restricts the dynamic range of the imager. Specifically, an excessive amount of background radiation causes the photodetectors to generate so much charge as to saturate the charge carrying capacity of the CCD registers, thus causing the image to be washed out. Such an imager can operate over only a very limited range of background radiation levels. One prior art solution to this problem was to interpose a gain control device between each photodetector and its corresponding column CCD register. Such a gain control device could be adjusted to limit the amount of charge generated by the photodetector for a given amount of incident radiation. For example, if the background radiation level were to increase for some reason, the user could adjust the gain control device to decrease the detector gain in order to avoid saturating the charge coupled device capacity. This solution has the disadvantage that such a reduction in detector gain simultaneously causes a reduction in the signal-to-noise ratio in the output video signal for high background radiation levels, due to photon noise. For example, in accordance with well-known principles, if the detector current is reduced by one-half, the signal-to-photon noise ratio will be reduced by a factor of (1/2).sup.1/2. Accordingly, in the prior art, saturation due to high intensity background radiation was avoided only at the expense of reduced signal-to-photon noise ratio.
Another problem with such imagers is that the clocking speed of the vertical column CCD parallel registers is limited by the clocking speed of the horizontal multiplexing CCD register to which they are all connected. Specifically, the clock frequency of the horizontal multiplexing register must be greater than the clocking frequency of the parallel vertical column registers by a factor proportional to the number of vertical columns in the array. The clock frequency of the horizontal multiplexing register is limited by the rise time of the CCD clock signal and the maximum speed the charge can be transferred, which places a fundamental limitation on the speed of such an imager. Also, transfer noise sets a limit on the performance of these devices due to the large number of transfers required to reach the output.
In summary, the dynamic range of a CCD imager is limited at extremely low background radiation intensities (such as those encountered in deep space) by the presence of thermal noise and row correlated noise. It is limited at very high background radiation intensities (such as those encountered on the ground in broad daylight) by the degradation of the signal-to-photon noise ratio caused by gain attenuation necessary to compensate for limited CCD charge capacity.